1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an In-Plane Switching (IPS) mode LCD device to improve the contrast ratio and to efficiently operate both reflective and transmitting modes.
2. Discussion of the Related Art
Recently, a liquid crystal display (LCD) device, which is a flat display, has been actively studied and researched because of its advantageous characteristics. The LCD device uses an optical anisotropy characteristic of liquid crystal to adjust light transmittance and display images. The optical anisotropy of the liquid crystal may be changed by applying an electric field to liquid crystal, which has both the fluidity of liquid and optical characteristics. The LCD device has attracted great interest as a new display device that can substitute for convention cathode ray tube (CRT) displays in that the LCD device has a thin profile, light weight, and low power consumption.
In general the LCD device includes a color filter (C/F) array substrate and a thin film transistor (TFT) array substrate, wherein the TFT and C/F substrates are positioned opposite to each other. Also, a liquid crystal layer having dielectric anisotropy is formed between the lower and upper substrates. The LCD device includes a plurality of pixels, each pixel having a thin film transistor TFT. A voltage is applied to the corresponding pixel through a pixel-selection address line by switching the thin film transistor of the pixel region.
The LCD device has various modes according to the properties of liquid crystal and pattern structures. Some of the various modes include a Twisted Nematic (TN) mode in which liquid crystal directors are controlled by applying a voltage after arrangement of liquid crystal directors twisted at 90°; a multi-domain mode in which a wide viewing angle may be obtained by dividing one pixel into several domains; an Optically Compensated Birefringence (OCB) mode in which a phase change of light is compensated according to the direction of light by forming a compensation film on an outer surface of a substrate; and an In-Plane Switching (IPS) mode in which a transverse electric field is created substantially parallel to the substrates by forming two electrodes on any one substrate.
In the meantime, LCD devices may also be classified as a transmitting type LCD, which uses a backlight as a light source; a reflective type LCD device, which was the ambient light as a light source; and a trans-reflective type LCD device, which uses both a backlight and the ambient light. In case of the trans-reflective type LCD device, it is possible to reduce the disadvantages of the transmitting type LCD device and the reflective type LCD device. That is, the transmitting type LCD device has the problem of high power consumption due to the use of backlight. Also, the reflective type LCD device can not be used in the dark surroundings.
The trans-reflective type LCD device includes unit pixel regions, wherein each of the unit pixels has a transmitting part and a reflective part. Thus, the trans-reflective type LCD device can use both the ambient light and the light generated from the backlight.
In the transmitting part of the transmitting type and trans-reflective type LCD devices, the light that is emitted from the backlight and is incident through the lower substrate reaches the liquid crystal layer, thereby improving the luminance. Also, in case of the reflective part of the reflective type and trans-reflective type LCD devices, in bright surroundings, the ambient light that is incident through the upper substrate is reflected so as to improve the luminance.
To realize the maximum efficiency in the transmitting and reflective parts, a dual-cell gap method has been proposed, in which a cell gap of the transmitting part is about twice as large as a cell gap of the reflective part.
The trans-reflective type may be applicable to the IPS mode LCD device. In this case, it is possible to maximize the efficiency in trans-reflective mode by forming an electrode of a dual-cell gap method.
Hereinafter, an IPS mode LCD device of a trans-reflective type will be described with reference to the accompanying drawings.
FIG. 1 is a plane view of an IPS mode LCD device according to the related art. FIG. 2 is a cross sectional view along I-I′ of FIG. 1. FIG. 3 is an optical schematic view according to the related art. FIG. 4 is a comparative table of showing the change of polarizing state in reflective and transmitting parts according to the related art.
Referring to in FIG. 1 and FIG. 2, an IPS mode LCD device according to the related art includes a plurality of pixel regions. Each pixel region includes of a reflective part R and a transmitting part T. The IPS mode LCD device includes a thin film transistor array substrate 11 including a plurality of lines and thin film transistors, a color filter array substrate 21 formed in opposite to the thin film transistor array substrate 11, and a liquid crystal layer 31 formed between the thin film transistor array substrate 11 and the color filter array substrate 21. In this state, the liquid crystal layer in the transmitting part (d1) is twice as large as a gap of liquid crystal in the reflective part (d2), which is referred to as a dual-cell gap structure.
As illustrated in FIGS. 1 and 2, the thin film transistor array substrate 11 includes a gate line 12, a data line 15, a thin film transistor TFT, a reflective plate 60, a passivation layer 16, a common electrode 24, and a pixel electrode 17. At this time, the gate line 12 is perpendicular to the data line 15, to define a unit pixel region. The thin film transistor TFT is formed near a crossing of the gate line 12 and the data line 15. The thin film transistor TFT includes a gate electrode 12a, a gate insulating layer 13, a semiconductor layer 14, and source and drain electrodes 15a and 15b. A reflective plate 60 is formed in the reflective part R, so as to reflect the ambient light. A passivation layer 16 corresponds with the data line 15 and the reflective part 60. Also, the common electrode 24 and the pixel electrode 17 are formed on a portion of the passivation layer 16, wherein the common electrode 24 and the pixel electrode 17 generate a transverse electric field.
The reflective part R of the IPS mode LCD of FIG. 1 includes a gate insulating layer 13 and the passivation layer 16. However, the gate insulating layer 13 and the passivation layer 16 are removed for the transmitting part, thereby forming a dual-cell gap. That is, the cell gap of liquid crystal in the transmitting part (d1) is twice as large as the cell gap (d2) of liquid crystal in the reflective part.
By removing the gate insulating layer 13 and the passivation layer 16 from the transmitting part, it is possible to maximize the efficiency of transmitting mode by appropriately applying turning-on and off modes in the transmitting and reflective parts. The cell gap ‘d1’ of the transmitting part T is twice as large as the cell gap ‘d2’ of the reflective part R.
Accordingly, the ambient light incident on the reflective part and the light incident on the transmitting part simultaneously reach the surface of screen for displaying images. That is, the ambient light incident on the reflective part reaches the surface of screen after passing through the liquid crystal layer twice. In the meantime, the light, emitted from the backlight and is incident on the transmitting part, reaches the surface of screen after passing through the liquid crystal layer of the transmitting part which has the cell gap twice as large as the reflective part. As a result, the light incident on the reflective part and the light incident on the transmitting part simultaneously reach the surface of screen for displaying images.
The reflective plate 60 is formed of aluminum Al, aluminum neodymium AlNd, or argentums Ag. The reflective plate 60 reflects the ambient light in bright surroundings, thereby displaying the image on the screen.
The transmitting part T includes a first portion from which the passivation layer 16 is removed, and a second portion in which the passivation layer 16 is formed. In the transmitting part T including the first and second portions, there are the first common electrode 24a and the pixel electrode 17, thereby forming a first transverse electric field E1. The pixel electrode 17 is also formed in the reflective part, whereby the pixel electrode 17 and the second common electrode 24b, provided on the passivation layer 16, generate a second transverse electric field E2.
In the IPS mode LCD device having the dual-cell gap by dividing the pixel region into the reflective and transmitting parts, the common and pixel electrodes are respectively formed in parallel to the transmitting and reflective parts. In the transmitting part, the first transverse electric field E1 is formed in the entire cell gap ‘d1’ of the transmitting part by the first common electrode 24a and the pixel electrode 17. In the reflective part, the second transverse electric field E2 is formed in the entire cell gap ‘d2’ of the reflective part by the second common electrode 24b and the pixel electrode 17. Accordingly, when the ambient light is not enough to drive the IPS mode LCD device, the IPS mode LCD device operates in the transmitting mode by the first transverse electric field E1. When the ambient light is enough to drive the IPS mode LCD device, the IPS mode LCD device operates in the reflective mode by the second transverse electric field E2.
The widths of the transmitting and reflective parts may be varied according to the size and number of pixel regions. Preferably, the ratio of width in the transmitting part to the reflective part is about 1 to 1 or 3 to 1.
The color filter array substrate 21 includes a black matrix layer 22, and a color filter layer 81. At this time, the black matrix layer 22 having a plurality of black matrix patterns is provided to prevent the light leakage. The color filter layer 81 includes a plurality of color filter patterns, wherein each of the color filter patterns is provided between each of the black matrix patterns.
In addition, first and second alignment layers (not shown) are formed on inner surfaces of the thin film transistor array substrate 11 and the color filter array substrate 21, so as to align liquid crystal molecules of the liquid crystal layer 31 in a predetermined direction. Also, first and second polarizing sheets 50 and 51 are formed on outer surfaces of the thin film transistor array substrate 11 and the color filter array substrate 21. A phase-difference plate is additionally formed between the color filter array substrate 21 and the second polarizing sheet, wherein the phase-difference sheet im parts phase delay.
The first and second polarizing sheets 50 and 51 transmit only the light parallel to the light-transmission axis, whereby the ambient light is changed to the linearly polarized light. The phase-difference plate changes the polarizing state of light, which is formed of a half wave plate HWP having a phase difference of λ/2 to change the linearly polarized light by the phase delay of 180°.
The transmission axes of the first and second polarizing sheets, the transmission axis of phase-difference plate, and the director of liquid crystal molecule may be configured so that the exemplary IPS LCD is in a normally black mode.
As illustrated in FIG. 3, the transmission axis of the phase-difference plate HWP is positioned at an angle of +Θ in relation to the transmission axis of the upper polarizing sheet 51 (upper POL). Also, the transmission axis of the lower polarizing sheet 50 (lower POL) is positioned at an angle of +Θ in relation to the optical axis of the phase-difference plate HWP. Then, the liquid crystal molecules are initially aligned at an angle of +45° in relation to the transmission axis of the lower polarizing sheet (lower POL). When the liquid crystal molecules are driven by an electric field, the liquid crystal molecules are rotated at an angle of −45° in relation to the transmission axis of the lower polarizing sheet, thereby realizing the white level.
The light path of a trans-reflective IPS LCD device with the optical structure of FIG. 3, will be described with reference to FIG. 4. In FIG. 4, arrows represent the direction of light passing through the respective parts.
In an OFF state in the reflective part, (i.e., when the liquid crystal is not driven), the ambient light incident on the upper polarizing sheet 51 (upper POL) is rotated at an angle of 2 Θ by the phase-difference plate HWP, and then the light passes through the liquid crystal, whereby the light is changed to the circularly polarized light. Thus, the circularly polarized light reaches the reflective plate. Then, the circularly polarized light is reflected on the reflective plate, and then the reflected light passes through the liquid crystal layer, whereby the light is changed to the linearly polarized light. Then, the linearly polarized light is rotated at an angle of 2 Θ by the phase-difference plate, whereby the light is emitted at an angle of 90° in relation to the transmission axis of the upper polarizing sheet 51. However, the light does not pass through the transmission axis of the upper polarizing sheet, thereby realizing the black level.
At this time, the cell gap of liquid crystal in the reflective part corresponds to ‘d/2’ (=Δ nd) corresponding to λ/4 (Quarter Wave Plate; QWP), whereby the linearly polarized light is changed to the circularly polarized light, and the circularly polarized light is changed to the linearly polarized light.
In an ON state in the reflective part, (i.e., when the liquid crystal is driven), the ambient light incident on the upper polarizing sheet 51 (upper POL) is rotated at an angle of 2 Θ by the phase-difference plate HWP, and then the light passes through the liquid crystal layer. After that, the light reaches the reflective plate. Then, the light is reflected on the reflective plate, and the reflected light passes through the liquid crystal layer. Therefore, the light is rotated at an angle of 2 Θ by the phase-difference plate HWP, whereby the light is emitted in the same direction as the transmission axis of the upper polarizing sheet 51. As the light passes through the upper polarizing sheet, it is realized as the white level. At this time, in case of driving the liquid crystal, the liquid crystal is rotated at an angle of −45°, whereby the liquid crystal is aligned in the same direction as the transmission axis of the lower polarizing sheet.
In case of the transmitting part, when the liquid crystal is not driven (off state), the polarizing direction of light emitted from the backlight and incident on the lower polarizing sheet 50 (lower POL) is changed to 90° by the liquid crystal molecules initially aligned. Then, the polarizing direction of light is changed at an angle of 2 Θ by the phase-difference plate HWP, whereby the light is emitted at an angle of 90° in relation to the transmission axis of the upper polarizing sheet 51. Accordingly, the light does not pass through the upper polarizing sheet, thereby realizing the black level.
At this time, the gap of liquid crystal in the transmitting part corresponds to ‘d’ (=2 Δ nd) corresponding to λ/2 (Half Wave Plate; HWP), to change the polarizing direction of light. That is, the polarizing direction of light is changed symmetric to the alignment direction of liquid crystal.
In the transmitting part, when the liquid crystal is driven (on state), the light, which is emitted from the backlight and is incident on the lower polarizing sheet 50 (lower POL), passes through the liquid crystal, and then the polarizing direction of light is changed by the phase-difference plate HWP, whereby the light is emitted in the same direction as the transmission axis of the upper polarizing sheet 51, thereby realizing the white level. At this time, in case of driving the liquid crystal, the liquid crystal molecules are rotated at an angle of −45°, whereby the liquid crystal molecules are aligned in the same direction as the transmission axis of the lower polarizing sheet.
Unlike the transmitting part of the transmitting type IPS mode LCD device, the transmitting part of the trans-reflective type IPS mode LCD device may have the circularly polarized light due to birefringence of the phase-difference plate HWP, thereby generating the luminance in the black level. Thus, it is impossible to realize a strong black level state in the IPS mode LCD device.